Softness of sanitary tissue, such as facial tissue and/or toilet tissue, and fibrous structures incorporated therein is of paramount importance. The purpose of being soft is so that these products can be used to cleanse the skin without being irritating. Making soft tissue products which promote comfortable cleaning without performance impairing sacrifices has long been the goal of the engineers and scientists who are devoted to research into improving tissue paper. There have been numerous attempts to reduce the abrasive effect, i.e., improve the softness of tissue products.
One area, which has received a considerable amount of attention, is the addition of chemical softening agents (also referred to herein as “chemical softeners”) to sanitary tissue products.
Because of the well known negative side effects associated with adding chemical softening agents to the wet end of the papermaking process, the addition of chemical softeners to a tissue paper fibrous structure (web) after the fibrous structure is dewatered, usually after it is partially or entirely dried, has received attention.
Many of these problems would be overcome if one could use a simple system to spray a functional additive directly onto the surface of the dried paper web just prior to winding. However, there are a number of problems associated with the use of spray systems for applying functional additives to a web and it has not been possible to obtain an even, complete coverage of functional additives onto a paper web at machine speeds. Traditionally, in the printing and writing paper and packaging paper industries, coating material is sprayed by pressure type nozzles, which employ the fluid pressure to disperse the fluid, creating large droplets of liquid, resulting in spotty coverage of the web. Typical spray systems used in the industry propel the fluid at a high velocity, generating sufficient force to cause a ricochet effect when the fluid impacts on the web resulting in a spotty uneven finish. With typical high pressure application, the center of the stream is more concentrated causing streaks on the coated surface while the outer edges of the spray fan are lost to the atmosphere, with a typical transfer efficiency of less than 50%. The outer edges of the fan may also dry before reaching the substrate, contributing to the poor transfer efficiency. The poor transfer efficiency may also contribute to equipment contamination as overspray is carried in the air, mixes with dust released from the paper web and the resulting mixture deposits on any surface that it may come into contact with, thereby contaminating the equipment and work environment.
In the case of the combination of a delicate web and a high viscosity additive, such as between about 50 cP and about 5000 cP, the needs for hygiene are particularly enhanced owing to the mixture of dust and functional additive elevating the hygiene impacts to a new level. The mixture of dust and functional additive is immediately apparent in any attempts to use conventional spray technology directly onto a dry, delicate web. The mixture of dust and functional additive is easily formed and has a marked impact on the reliability of the operation. Researchers use the term “kgnarr” to refer to this contaminant formed when a functional chemical additive unites with the dust in the surroundings of the traveling web in an additive-application area. Elimination of kgnarr is essential to achieving a reliable application of a functional chemical additive onto a delicate fibrous structure during the papermaking process.
Accordingly, there is a need for a simple, flexible and efficient method for applying a chemical additive, such as a chemical softener, to a fibrous structure (web) while the fibrous structure is moving, typically at a high speed e.g., greater than about 100 m/min, without the creation of kgnarr.